Certain surgeries, particularly coronary artery bypass surgery, necessarily involve the use of suture needles of small diameter having exceedingly high bending stiffness and strength. In particular, surgery of this type requires that the suture needle's path be closely controlled. If the needle flexes excessively as it enters the tissue or as it pierces the inner surface of e.g., a blood vessel before re-emerging, improper placement of the needle and serious trauma to the tissue and the patient can occur. In use, suture needles are subjected to substantial stressing forces, since the force used to drive the needle into and through tissue (e.g., a blood vessel and the like) needs to be sufficient to overcome frictional drag through the tissue. These forces resisting needle penetration are commonly exacerbated in patients undergoing cardiovascular surgery, who exhibit calcified or toughened tissue due to coronary artery disease. In these procedures, the suture needle must be able to pass through not only the blood vessel, but also any hard calcified tissue that may be located along the periphery of the blood vessel lumen. A compliant needle will deflect elastically during tissue penetration resulting in a loss of placement control. As such, it is preferable that the needle should have a relatively high bending stiffness, that is, a low tendency to flex and high tendency to retain its configuration when subjected to a deforming force. Hence, stiffness in bending is an essential property for the handling and performance of suture needles. A stiff needle resists elastic deflection and can thus be directed as intended to provide a high level of control.
ASTM standard F1840-98a (Reapproved 2004) provides standard terminology for surgical suture needles and ASTM standard F1874-98 (Reapproved 2004) provides details of a standard test method for bend testing of needles used in surgical sutures. Both ASTM standards are incorporated herein by reference. Two different measures for the strength of surgical suture needles are used, namely, yield bend moment, which is the amount of moment required to initiate plastic deformation during a bend test, and maximum bend moment, which is the greatest moment applied to a needle during a bend test. This later value of maximum bend moment is typically measured at a point where the needle has undergone substantial plastic deformation and is generally higher than the yield bend moment or point at which plastic deformation initiates. The point of deflection at which plastic deformation initiates, or more formally according to ASTM standards, the angle at which the yield bend moment occurs, is referred to as the yield bend angle.
Both needle bending strength and needle bending stiffness influence handling characteristics, as well as penetration performance and efficacy of the suture needle. It is important to note that in almost all circumstances, the suture needle should be used in applications where the yield bend moment is not exceeded, since above this value, the needle will bend plastically, losing its original shape, and will no longer function as intended. It is thus apparent that a desirable characteristic of a suture needle is a high yield bend moment, which is a manifestation of the bending strength of the suture needle. Below the yield bend moment, the resistance of bending of the suture needle is best characterized by the needle bending stiffness. Needle bending stiffness is a critical measure of the resistance to elastic, or recoverable bending of the suture needle before needle deflection reaches the yield bend angle and can be calculated as the yield bend moment divided by the yield bend angle. If a straight or curved suture needle has a low value of bending stiffness, substantial bending of the needle will occur for a given bend moment, whereas if a straight or curved suture needle exhibits a high bending stiffness value, relatively little elastic bending of the needle will occur for a given bend moment. Surgeons will tend to perceive a high degree of elastic bending as a loss of control or as a poor penetration performance since the needle point is not translating directly with the motion of their hands. As such, needle bending stiffness may be recognized as a quintessential measure of needle performance in most surgical applications.
Hence, the desirable bend properties for a suture needle are high bending stiffness, as well as bending strength manifested as high yield bend moment and ductility, in order to penetrate tissue which is being sutured without undue flexing, plastic bending, or breaking during a surgical procedure.
The needle should also not be brittle; if any portion of the needle is too brittle it may break during use if too much force is applied. The needle should instead be ductile, which is the ability to bend without breaking. Curved suture needles are commonly bent through a bend angle of 90 degrees and then manually reshaped to their original curvature to assess ductility. Those skilled in the art of needle making will recognize this procedure as the reshaping process and will further recognize that the higher the number of reshape processes that a needle can withstand without breaking the more ductile it is.
U.S. Pat. No. 5,415,707 describes tungsten alloy surgical needles that exhibit high tensile yield strength in excess of 250,000 psi, high tensile modulus of elasticity or stiffness in excess of 45×106 psi, and high ductility. The needles described therein preferably comprise about 3 to about 6 weight percent of rhenium, rhodium and/or iridium. Data presented in U.S. Pat. No. 5,415,707 was derived from straight uncurved needles.
As described in U.S. Pat. No. 5,415,707, tungsten alloys have exceptionally high stiffness along with other desirable physical properties. Tungsten alloys derive their strength from their high dislocation density and the natural resistance to deformation that occurs via dislocation-dislocation interaction as a stress is applied. However, the exceptionally high stiffness of such tungsten alloys in wire and straight needle form does not necessarily translate to high bending stiffness when such alloys are used to make curved suture needles, since the curving process during needle manufacture imparts stresses that act to reduce the bending stiffness of the curved suture needle. It is believed that during the curving portion of the needle manufacture process, the dislocations in the tungsten alloy move to high energy locations within the microstructure, or locations where high strain field exist locally around the dislocations. When a moderate unbending force is applied to the curved suture needle, the dislocations in the high energy locations readily slip to positions of lower energy or lower local strain. The slip of these dislocations to lower energy positions manifests itself as limited plastic deformation, resulting in a relatively low stiffness in bending or low yield bend moment.
Thus, there is a need for tungsten alloy suture needles that exhibit high bending strength and high bending stiffness, particularly when the suture needle is a curved needle.